Processes for hydrogenating heavy hydrocarbon oil (hereinafter referred to simply as heavy oil) containing large amounts of asphaltene and heavy metals such as nickel and vanadium to convert the oil into desulfurized hydrocarbon oil having a low asphaltene content and increased value have been intensively studied.
When heavy oil is hydrogenated, the loading of poisoning materials per unit quantity of catalyst is high due to the presence of asphaltene or heavy metal compounds at high concentrations in the heavy oil. As a consequence, the activity of the catalyst is rapidly decreased. Asphaltene which is dispersed as colloidal particles in the heavy oil is a macromolecule. When conventional hydrodesulfurization catalysts are used, asphaltene creates high resistance to diffusion of oil in catalyst particles and coke is formed in great quantities on the surfaces of the catalyst particles, catalyst activity is rapidly decreased and the desired reactions are hindered in a short period of time. Further, heavy metals such as nickel and vanadium in the heavy oil are deposited on the surfaces of the catalyst particles and the catalyst is also poisoned by such heavy metals, that is, catalyst activity is rapidly decreased. Therefore, the catalyst must be replaced with fresh catalyst and cost is increased. In some cases, heavy oil is no longer treated by refinerys.
In carrying out the hydrodesulfurization of heavy oil of relatively good quality having an asphaltene content of not higher than about 2% and a heavy metal content of not higher than about 50 ppm with conventional processes, hydrodesulfurization catalysts having pores of a relatively small pore size of about 60 to 110 .ANG. are used. (Unless otherwise indicated, all percent and ppm are by weight.) In the hydrodesulfurization of heavy oil containing large amounts of asphaltene and heavy metals such as heavy oil having an asphaltene content of as high as more than about 3% and a heavy metal content of as high as more than about 50 ppm, poisoning materials are deposited due to the presence of asphaltene (a high molecular weight colloid) or the heavy metal compounds in substantial quantities, catalyst activity is decreased and the catalysts are deteriorated to such an extent that they can be no longer used.
When the deteriorated catalysts are taken out and analyzed, it is found that vanadium and nickel are deposited on the surface layer of the catalyst particles, pores on the surface layer are clogged and heavy oil cannot diffuse into the interior of the catalyst particles. Accordingly, it is necessary to adjust the pore distribution of the catalyst and to improve the shape of molded catalyst particles to allow the diffusion and penetration of the heavy oil into the interior of the catalyst particles to be accelerated despite the presence, of asphaltene or the content and molecular structure of the heavy metal compounds which poison the catalyst in the hydrogenation of heavy oil.
Attempts have been made to charge catalysts which have a large pore size for removing only metals in the first stage of a reactor, this stage accounting for 10 to 50% of the total capacity of the reactor, to overcome the above problem. However, this approach involves a serious disadvantage: since the pore size of the catalyst charged into the first stage is large, the active surface area of the catalyst is small and the desulfurization capability in the reaction is low from the start.
In JP-B-47-40683 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-54-125192 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-57-201533, there are proposed catalysts wherein passages only for diffusion are provided in the interior of the catalyst particles to permit raw oil to diffuse into the interior of the catalyst particles without greatly enlarging pore size.
In JP-B-47-40683, large-diameter passages having a size of 100 to 1000 .ANG. or larger only for diffusion account for 20 to 30% of the total pores, and in the other two applications, large diameter passages having a size of about 500 .ANG. or larger account for about 10 to 30% of the total pores so that the life of the catalyst is prolonged and desulfurization ability is improved, even when metals are deposited. However, the large diameter passages only for diffusion are themselves merely "spaces" which have no reaction activity and such spaces account for 10 to 30% of the whole of the pores. The provision of such spaces only for diffusion reduces the area of catalytic activity in the reactor which should be as high as possible.
Accordingly, to solve the above problems, there has been a great demand to discover a hydrogenation catalyst which is excellent in desulfurization performance as well as in metal removal activity, that is, which exhibits both desulfurization and metal removal functions.